I'd recommend just buying it from Blizzard directly. It's on sale for $30 right now, and it upgrades your account immediately.

If you want to shop around, stick to well-known retailers like Amazon. I wouldn't trust sites that look like the one you linked. It could be legit, but it's more likely it's a scam of some kind; either they'll sell your credit card info, or they'll just take your money, never send you anything, and string you along for months hoping you'll just give up and they'll get to keep your money.